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Are Carbonate Barrier Islands Mobile?

Abstract

Today barrier islands occur mostly along siliciclastic shorelines. Such islands are recognized as mobile landforms that are constructed during transgressions and migrate inboard with sea-level rise. Carbonate barrier islands, on the other hand, are generally not understood to be mobile. The barriers of the “Great Pearl Bank”, United Arab Emirates, are interpreted to be nucleated on, and anchored in place by, Pleistocene eolian deposits. Likewise, many platform-margin carbonate islands (e.g. the Exuma Island chain) are cored by meteorically-cemented Pleistocene rocks, and flanked by coral frameworks, and so are also immobile relative to siliciclastic barrier islands. Furthermore, generation of grainy sediment in the high-energy factories of aggraded platform margins is genetically-linked to platform-margin tidal-current amplification. This sediment feeds subaqueous, margin-normal sandbars, rather than mobile barrier islands. Carbonate barrier islands formed along the northern coastline of Qatar, conversely, are identical to siliciclastic barrier island systems with regard to both geomorphic components and the apparent processes of formation. Field investigations indicate that these landforms have migrated one km or more in both landward and seaward directions during the middle to late Holocene, behaving identically to siliciclastic barriers. Inspection of Qatar coastlines to the south shows similar near-shore barrier/back-barrier deposits have filled kilometers of shallow-marine accommodation over the past 6000 years in association with a sea-level fall of < 2 m. Despite the Qatar examples, mobile carbonate barrier islands are rare in modern neritic warm-water settings. Their construction and migration may be inhibited by reef formation, early meteoric cementation, the greater influence of tide-related sediment redistribution relative to wave-related distribution at platform margins, and the relative inefficiency of sourcing beach sediments from the open shelf. These inhibiting factors are minimized in neritic, cool-water realms, and carbonate barrier islands are correspondingly more common. Given these observations, barrier islands were likely more abundant during geologic periods when ramps and unrimmed shelves predominated (non-reefal periods) and in calcite seas, when meteoric cementation was diminished.